For a sub-set of extremely sensitive data, things like your social security number, passport and such, Nextcloud now introduces end-to-end encryption. The Nextcloud end-to-end encryption feature is designed such that the server never has access to unencrypted files or keys, nor does server-provided code ever handle unencrypted data which could provide avenues for compromise. You pick one or a few folders and encrypt them locally on the client, thus protecting them from any breach that might occur on the server.

Learn more on https://nextcloud.com/endtoend